vsock/test: verify socket options after setting them

Replace setsockopt() calls with calls to functions that follow
setsockopt() with getsockopt() and check that the returned value and its
size are the same as have been set. (Except in vsock_perf.)

Signed-off-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>